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Discharging construction wastewater into public waterways

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The European Parliament Water Framework Directive (WFD), 2000/60/EC, is a standard establishing a common framework for water policy in Europe. Its aim is to ensure the protection of water and promote its sustainable use to ensure the long-term availability of this natural resource. The WFD takes water from being a simple resource to being considered in the European Union as the key to the conservation of living systems associated with it. It represented a milestone in the management of water resources and related ecosystems in Europe and was a reference for most developed countries.

Thus, the reduction of water consumption and proper treatment of wastewater from industrial production processes, either for recycling or disposal, are basic elements for proper water management in a company.

In the case of the construction industry, wastewater from a site usually contains very high levels of suspended solids (SS) and fats or oils from machinery as major pollutants, and sometimes different types of nitrates from explosives, for example. If not treated properly, this type of water can produce a very high risk, especially for river flora and fauna, as well as for humans. However, in many cases, after treating and purifying the water from the site, it can be re-used as part of the subsequent optimisation of resources.

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The existing water standards in major developed countries are intended to regulate its discharge, both as sewage and into rivers or the sea, while establishing the required quality standards in each case.

The discharge of wastewater into the public water domain requires compliance with the quantitative and qualitative limits established by the local authority or government; and only discharges that comply with authorised limits are considered legal. For example, in the Júcar river basin in Spain, the following minimum concentration requirements for secondary treatment are established:

Parameter Concentration in effluent
BOD5 25 mg/L O2
COD 125 mg/L O2
SS 35 mg/L (> 10,000 i.-e)
SS 60 mg/L (2,000 – 10,000 i.-e)

where i.-e = inhabitant equivalent, or biodegradable organic load with a biochemical oxygen demand for 5 days (BOD5) of 60g of oxygen per day

As well as being discharged directly into inland or marine waters, wastewater can also be discharged into a sewage or collection system or wastewater treatment plant (WWTP). A collection system is any system of conduits which collects and conducts urban wastewater from municipal-owned sewage networks into treatment or sewage stations. If discharged into the municipal sewer, the permit for this is granted by the local council.

The discharge limits for municipal sewer systems are decided individually in the corresponding disposal regulations. The table below shows a typical example of the wastewater discharge limits into sewers for such local ordinances:

Parameter Daily mean concentration maximum
pH 5.50 – 9.00
Suspended solids (mg/L) 500
Settleable solids (ml/L) 15
Coarse solids Absent
DBO5 (mg/L) 500
COD (mg/L) 1000

As well as other maximum values in mg/L of chemical elements such as arsenic, boron, cadmium, chromium, iron, mercury, lead, selenium, nickel and zinc.

Thus, a detailed prior assessment of any industrial or construction project is required, for management of the wastewater to be efficient for the company and safe for people and the environment.

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